Reset lastOverflowedXid on standby when needed

Currently, lastOverflowedXid is never reset.  It's just adjusted on new
transactions known to be overflowed.  But if there are no overflowed
transactions for a long time, snapshots could be mistakenly marked as
suboverflowed due to wraparound.

This commit fixes this issue by resetting lastOverflowedXid when needed
altogether with KnownAssignedXids.

Backpatch to all supported versions.

Reported-by: Stan Hu
Discussion: https://postgr.es/m/CAMBWrQ%3DFp5UAsU_nATY7EMY7NHczG4-DTDU%3DmCvBQZAQ6wa2xQ%40mail.gmail.com
Author: Kyotaro Horiguchi, Alexander Korotkov
Reviewed-by: Stan Hu, Simon Riggs, Nikolay Samokhvalov, Andrey Borodin, Dmitry Dolgov

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index a99c5b93e9e6886fa907da09d24d761a698854db..5798c12a44a44bfcb676b7ff5b86b3dc37b01915 100644 (file)
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -3290,24 +3290,41 @@ ExpireTreeKnownAssignedTransactionIds(TransactionId xid, int nsubxids,
 
 /*
  * ExpireAllKnownAssignedTransactionIds
- *     Remove all entries in KnownAssignedXids
+ *     Remove all entries in KnownAssignedXids and reset lastOverflowedXid.
  */
 void
 ExpireAllKnownAssignedTransactionIds(void)
 {
    LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    KnownAssignedXidsRemovePreceding(InvalidTransactionId);
+
+   /*
+    * Reset lastOverflowedXid.  Currently, lastOverflowedXid has no use after
+    * the call of this function.  But do this for unification with what
+    * ExpireOldKnownAssignedTransactionIds() do.
+    */
+   procArray->lastOverflowedXid = InvalidTransactionId;
    LWLockRelease(ProcArrayLock);
 }
 
 /*
  * ExpireOldKnownAssignedTransactionIds
- *     Remove KnownAssignedXids entries preceding the given XID
+ *     Remove KnownAssignedXids entries preceding the given XID and
+ *     potentially reset lastOverflowedXid.
  */
 void
 ExpireOldKnownAssignedTransactionIds(TransactionId xid)
 {
    LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
+
+   /*
+    * Reset lastOverflowedXid if we know all transactions that have been
+    * possibly running are being gone.  Not doing so could cause an incorrect
+    * lastOverflowedXid value, which makes extra snapshots be marked as
+    * suboverflowed.
+    */
+   if (TransactionIdPrecedes(procArray->lastOverflowedXid, xid))
+       procArray->lastOverflowedXid = InvalidTransactionId;
    KnownAssignedXidsRemovePreceding(xid);
    LWLockRelease(ProcArrayLock);
 }